A MAN who died about 60,000 years ago in Australia may be about to turn our
theory of human origins on its head. Researchers in Australia have accomplished
the extremely difficult feat of extracting DNA from his skeleton, and were
astonished to find that it looks like nothing they have ever seen before.
The DNA, which is the oldest ever recovered from human remains, shows that
while the man is completely anatomically modern, he came from a genetic lineage
that is now extinct. This finding challenges the prevailing theory that all
modern humans are descended from a group of people who migrated from Africa
around 100,000 years ago. 鈥淚t鈥檚 remarkable鈥攖otally unpredicted,鈥� says
anthropologist Alan Mann of the University of Pennsylvania. 鈥淲hat it says is
that the more we know [about human origins], the more confusing the picture
产别肠辞尘别蝉.鈥�
Mungo Man鈥檚 remains were found on the shores of Lake Mungo in south-eastern
Australia in 1974. They were originally radiocarbon-dated to about 30,000 years
old, but in 1999 a reassessment using three different techniques showed the
bones to date from around 60,000 years ago.
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In 1995, a team led by anthropologist Alan Thorne of the Australian National
University in Canberra began an attempt to extract genetic material from the
remains. Doctoral student Gregory Adcock and his colleagues at CSIRO Plant
Industry managed to replicate and sequence a single gene from Mungo Man鈥檚
mitochondria, the powerhouses of cells whose small genome is passed down the
female line.
Simon Easteal, an evolutionary geneticist at ANU, then set about analysing
the sequence and comparing it with sequences of the same gene from nine other
early Australians鈥攔anging in age from 8000 to 15,000 years鈥攁s well
as 3453 contemporary people from around the world, chimpanzees, bonobos (pygmy
chimps) and two European Neanderthals.
Easteal looked for patterns of descent and worked out which 鈥済enetic tree鈥�
fitted the data best. According to this evolutionary tree, chimps and bonobos
were first to branch off the trunk leading to modern people. Neanderthals split
off next, then Mungo Man鈥檚 line and finally the line that led to the most recent
common ancestor of contemporary people, including the ancient Australians but
excluding Mungo Man. 鈥淲e can say with a high degree of confidence that modern
people arrived in Australia before the new lineage [of the most recent common
ancestor] arrived,鈥� Easteal says.
According to Thorne, the findings鈥攄ue to be published next week in the
online edition of the Proceedings of the National Academy of Sciences
鈥攖hreaten to topple the leading theory of human origins, the
鈥渙ut-of-Africa鈥� model. This proposes that all living people are descended from a
group of modern Homo sapiens who left Africa roughly 100,000 to 150,000
years ago. Their descendants spread around the world, replacing existing
populations of 鈥渁rchaic鈥� people, such as Neanderthals and the more ancient
Homo erectus.
But if anatomically modern humans鈥攆rom a lineage that emerged before
the most recent common ancestor of people today鈥攚ere living in Australia
60,000 years ago, 鈥渁 simplistic out-of-Africa model is no longer tenable鈥�, says
Thorne.
Thorne is one of the founders of the rival 鈥渞egional continuity鈥� model, which
postulates that H. erectus began migrating from Africa over 1.5 million
years ago, and from these migrants H. sapiens evolved at the same time
in various regions around the world. Those early people remained on the same
evolutionary path by sharing their genes through interbreeding.
In Thorne鈥檚 scenario, Mungo Man鈥檚 ancestors probably evolved in Asia. They
gradually migrated to Australia, where the lineage vanished. Because the lineage
is based on a single mitochondrial gene, it is too early to know exactly what
happened. The clan could have been wiped out by newcomers, or the gene may, for
some reason, not have been passed from mother to daughter.
That is why out-of-Africa proponents, including ANU physical anthropologist
Colin Groves, argue that the new data does not knock their model from the top of
the theoretical pile. The genetic evidence is equivocal, he says. 鈥淭he
African-origin model stands or falls by the fossil evidence. In my opinion, it
蝉迟补苍诲蝉.鈥�
But Groves praises Adcock鈥檚 technical achievement. Retrieving such old DNA is
a 鈥渞eal coup鈥�, he says.
